Printer system having a plurality of light sources of different wavelengths

ABSTRACT

A printer system for producing color prints includes: (a) three light sources, each generating a light beam of different wavelengths; (b) three modulators, each of the modulators modulating intensity of each of the light beams by image data, to provide three modulated light beams; (c) a beam combiner combining the three modulated light beams into a single light beam; (d) a single set of beam shaping optics shaping this single light beam into a shaped light beam having different vergences and waist locations in the page and line directions, the vergences and the waist locations being substantially the same in each of the three wavelengths; (e) a light deflector deflecting the shaped light beam to provide a deflected beam, and performing a scanning function; (f) an f-θ lens focusing the deflected light beam onto an associated image surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation-in-Part of application Ser. No. 08/841,443, filed Apr. 22, 1997, entitled "A Printer System" by Badhri Narayan et al. This application is also related to the following applications:

"A Printer System Utilizing Three Different Data Rates" by Badhri Narayan and Drew D. Summers, U.S. patent application Ser. No. 09,025,273, filed Feb. 18, 1998, which in turn is a Continuation-in-Part of U.S. Pat. application Ser. No. 08/841,442, filed Apr. 22, 1997.

"An F-θ Lens" by John D. Griffith, U.S. patent application Ser. No. 08/841,418, filed Apr. 22, 1997;

FIELD OF THE INVENTION

This invention relates to color printers, and more particularly, to color printers utilizing multiple laser sources producing light beams in different wavelengths.

BACKGROUND OF THE INVENTION

Laser printers are used in the photographic and graphic arts for printing on a photosensitive medium such as film or paper. When such printers are used for color imaging, they generally include three different light sources, each providing a light beam of a different wavelength. Light beams from these light sources are individually shaped by three separate sets of beam shaping optics (one set for each light beam), and are then combined into a single light beam with three coaxial components. This light beam is then scanned over an angular range by a rotating polygon with a plurality of reflective facets. An f-θ lens is located in the optical path between the polygon and the photosensitive medium. The single light beam, when incident on the polygon and the f-θ lens, would be typically collimated in each of the three wavelengths. Such printer is disclosed in U.S. Pat. No. 4,821,113. Not only does this printer have a separate set of beam shaping optics for each light beam, but it also operates in the infrared (I.R.) light region.

Because the printer is operating in I.R. region, this printer can not use conventional, silver halide based, photographic paper for printing images. Instead it has to use "false sensitized" paper that is matched to specific wavelengths of interest (i.e., 780 nm, 830 nm and 880 nm). More specifically, the conventional photographic paper is designed to be exposed with visible light and incorporates blue, green and red sensitive layers that produce yellow, magenta, and cyan dye layers in the final image. Laser printers designed to write with lasers that operate in the infrared region require, for their photosensitive medium, a special "false-sensitized" paper in which the red, green and blue sensitive layers are replaced by layers which are sensitive to three distinct infrared wavelengths. Development, production, and distribution of the "false-sensitized" paper is expensive.

U.S. Pat. Nos. 5,111,325 and 5,018,805 each disclose a color printer that operates with light beams at the wavelengths 750 nm, 810 nm, and 870 nm (I.R. region). This printer also utilizes three sets of beam shaping optics. More specifically, the f-θ lens disclosed in U.S. Pat. No. 5,111,325 operates with three light beams whose vergences are controlled independently of one another. The word "vergence", as used herein refers to whether the input light beam to the f-θ lens is collimated, converging or diverging. A collimated light beam has zero vergence, a diverging light beam has negative vergence and a converging light beam has positive vergence. The concept of "vergence" is described in more detail in column 4, lines 46-62 of the above mentioned U.S. Pat. No. 5,111,325. Since axial color is a focus related phenomenon, it is caused not only by aberrations of the f-θ lens itself, but also by the vergence of the input beam to the f-θ lens. In the printer disclosed in U.S. Pat. No. 5,111,325 and U.S. Pat. No. 5,018,805, the desired vergence in each of the three light beams is controlled by its own set of beam shaping optics located in each color channel. Therefore, at least three sets of independent beam shaping optics are required by this printer. These three sets of beam shaping optics provide the ability to control the vergences of the three light beams separately, which greatly reduces the problem of correcting axial color, but increases complexity and the cost of manufacturing and assembly.

In U.S. Pat. No. 4,728,965 there is disclosed a color printer having three light sources, each providing a visible wavelength light beam (red, green, blue, respectively). This printer can operate with photographic paper. However, this printer also has three sets of beam shaping optics (one for each channel).

In U.S. Pat. No. 5,475,415, there is disclosed a thermal printer with multiple light sources. In this type of printer, all of the light beams have the same wavelength. The multiple light sources are simply used to increase the throughput of the printer. Because all of the light beams have the same characteristics (wavelength and divergence) they can be shaped by a single set of beam shaping optics. However, such a configuration is not suitable for color printers using light beams with different wavelengths and different vergences because of chromatic aberrations.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the problems in the prior art discussed above and to provide an improved f-θ lens for a color laser printer.

According to the present invention the printer system comprises: (a) at least two light sources, each of the light beam sources producing a light beam of a different wavelength; (b) at least two modulators, one for each beam, the modulators modulating the light beams with image data, providing modulated light beams; (c) a beam combiner combining the modulated light beams into a single light beam having at least two coaxial components of different wavelengths; (d) a single set of beam shaping optics shaping this single light beam into a shaped light beam; (e) a light deflector deflecting the shaped light beam to provide a deflected beam; (f) an f-θ lens intercepting and focusing the deflected light beam on a photosensitive medium.

According to one embodiment of the present invention, a printer system comprises: (a) three light sources, each generating a light beam of different wavelengths; (b) three modulators, each of the modulators modulating intensity of each of the light beams by image data, to provide three modulated light beams; (c) a beam combiner combining the three modulated light beams into a single light beam; (d) a single set of beam shaping optics shaping this single light beam into a shaped light beam having different vergences and waist locations in page and line directions, the vergences and the waist locations being substantially the same in each of the three wavelengths; (e) a light deflector deflecting the shaped light beam to provide a deflected beam, and performing a scanning function; (f) and f-θ lens focusing the deflected light beam onto an associated image surface.

It is an advantage of the printer system of the present invention that it enables for color printing with ordinary photographic paper instead of prior art special paper.

Another advantage of this printer system is that it has only one set of beam shaping optics. Thus the printer system of the present invention can be manufactured at a lower cost than the prior art color printers.

A further advantage of the printer system of the present invention is that it is designed to operate with the green, blue, and red light beams combined by a fiber optic multiplexor. This provides a significant reduction in cost and results in compactness of the printer system.

Other features and advantages will become apparent with reference to the following description of the preferred embodiments when read in light of the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic illustration of an embodiment of the printer system including an f-θ lens.

FIG. 1b illustrates schematically the color separation along a scan line on the surface of a photosensitive medium.

FIG. 2 is a schematic illustration of how one of the light beams is directed to one of the modulators of the printer system of FIG. 1a.

FIG. 3 is a schematic illustration showing how light beams may be coupled to fibers and then directed to the modulators of the printer system of FIG. 1a.

FIG. 4 is a top plan view showing the lens element arrangement in the f-θ lens shown in FIG. 1a.

FIG. 5 is a schematic illustration of a composite beam waist formed at an output end of a beam combining fiber.

FIGS. 6 and 7 are schematic views showing a light beam with one set of waists, W₁, located in one plane and another set of waists, W₂, located in another plane.

FIG. 8a is a schematic elevational view showing the f-θ lens of FIG. 4 in combination with a plano mirror and a cylindrical mirror, and a deflected light beam going through the F-θ lens and striking the photosensitive medium.

FIGS. 8b-8d are three perspective views of the f-θ lens of FIG. 4, pre-polygon beam shaping and focusing optics, post-polygon cylindrical mirror, and an associated image surface.

FIG. 8e shows an embodiment of the post-polygon cylindrical mirror.

FIGS. 9a-9c are plan views of the f-θ lens, the piano mirror and the cylindrical mirror illustrated in FIG. 8a. More specifically, FIGS. 9a-9c show the path of the deflected light beam 42 for the polygon rotations of 0°,--13.5°, and +13.5 °, respectively.

FIG. 10 is a an aberration plot showing the optical path differences at the center of a scan line in all three wavelengths.

FIG. 11 illustrates schematically how different color beams intercept pixels at a given time T₁.

FIG. 12 is a schematic illustration showing different pixels at the photosensitive medium receiving red, green and blue light beams at different times.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Printer System

In the following discussion and throughout this application the term "page direction" means the cross-scan direction. It is the direction perpendicular to the scan line produced by a rotation of a polygon or other deflector. The term "line direction" means the direction of the scan line produced by the rotation of the polygon or other deflectors. These directions must be understood in the context of the local coordinate system of an optical component; the coordinate system will be tilted by fold mirrors. The optical axis of the printer system is the Z axis, the page direction is the X direction, and the line direction is the Y direction.

The overall printer system 10 illustrated in FIG. 1a includes: (a) a digital image store, containing three values for each pixel of at least a scan line that is being scanned, each of the three values representing the intensity required at one of three wavelengths to produce a correct color on an associated photosensitive medium, (b) three laser sources, each laser source producing a light beam of a different wavelength, (c) a means for individually modulating the intensity of each of the three light beams to provide three modulated light beams, (d) a means for combining the three modulated light beams into a single beam, (e) a single set of apochromatic beam shaping optics for shaping the single beam to create a shaped light beam having different vergences and waist locations in page and line directions, (f) a deflector such as a rotating polygon with a plurality of reflective facets, which deflects the shaped beam, (g) an f-θ lens which is corrected for primary and secondary axial color, but is uncorrected for lateral color, (h) a conjugating cylindrical mirror used in conjunction with the f-θ lens to correct pyramid error of the printer system simultaneously in all three wavelengths, (i) a means to support and transport the associated photosensitive medium in a direction perpendicular to the scan line created by the movement of the deflector. The associated photosensitive medium is a conventional photographic paper sensitive to light beams at the wavelengths produced by the above mentioned laser sources. According to one embodiment of the present invention, the photosensitive medium is a planar, that is, a sheet of photosensitive material.

The lateral color in the f-θ lens is corrected by reading the values from the digital image store, processing the values with a processor unit, and modulating the corresponding light beams with three modulators at three different data rates. The pixel data in the digital image store controls the intensities of the beams of the three lasers by controlling the modulators. The separation of the different color spots (FIG. 1b) is compensated for by moving data from the digital image store to the modulator control circuitry at three different data rates.

More specifically, the printer system 10 of FIG. 1a is provided with a plurality of light sources 22, 24 and 26. The light sources 22, 24 and 26 are laser sources that emit red, green and blue wavelength light beams 12, 14 and 16, respectively. These light beams 12, 14 and 16 are propagated to a plurality of acousto-optical modulators. In this embodiment the acousto-optical modulators 32, 34, and 36 are used as the light modulating means for modulating the light beams by image information. Acousto-optical modulators are well known devices. However, the light modulating means is not limited thereto. Other means for modulating the light beams may be employed.

Each of these acousto-optical modulators 32, 34, 36 modulates its associated light beam by changing its intensity according to the image data provided. This will be discussed in more detail in the "Lateral Color Correction" section of this specification. All three light beams are modulated simultaneously.

Two preferred embodiments are shown in FIGS. 2 and 3 to couple light beams 12, 14, 16 from the laser sources to the modulators. FIG. 2 shows that a laser light beam 12 is directed to the modulator 32 through a monochromatic focusing lens 31 to form a beam waist at the modulator. A similar arrangement is used for the light beams 14 and 16. FIG. 3 shows that, alternatively, the light beams 12, 14, 16 may be coupled to a single mode fiber through a fiber optic connector 23, 25, 27. The fiber optic connector comprises of a first focusing lens 23a, 25a, 27a, a fiber 23b, 25b, 27b, and a fiber holder 23c, 25c, 27c with a mechanical motion capability to precisely locate and maintain the position of the fiber with respect to the laser beam 12, so as to maximize the amount of light coupled into the fiber. The beam waist formed on the end of the fiber 23b, 25b, 27b is re-imaged by a second lens 23d, 25d, 27d to form an appropriate beam waist at the modulator 32, 34, 36. More specifically, the fiber 23b, 25b, 27b circularizes the beam and a circular beam waist is then formed at the modulator 32, 34, 36.

The modulated light beams are optically combined into a single light beam 42 by an optical combiner such as a conventional fiber optic multiplexor 40 (as shown in FIG. 1a). The fiber optic multiplexor 40 has appropriate fiber connectors (similar to fiber optic connectors 23, 25, 27) to couple the light beams exiting the modulators to the input fibers 40a, 40b, 40c of the fiber optic multiplexor 40. The fiber optic multiplexor 40 produces a beam waist of different size in each of the three colors at the output end of a beam combining fiber 40d (see FIG. 5). These beam waists are coplanar. In one embodiment the radii of these waists at the exp(-2) power level in this embodiment are: 0.00189 mm at λ=532 nm (green color G), 0.00172 mm at λ=457.9 nm (blue color B) and 0.00237 mm at λ=685 nm (red color R). The shapes of the beam waists formed at the output end of the beam combining fiber 40d are circular.

The combined beam (of red, blue and green components) exits the multiplexor 40 (at the output end of the fiber) as a single light beam 42 traveling along an optical axis 41 (which is common to the three components) and is directed first towards an apochromatic focusing lens 50, and then to a single set of beam shaping optics 52 (FIG. 1a). The focusing lens 50 re-images the three circular beam waists (red R, green G, blue B) produced at the output end 40d of the fiber to a second set of larger size beam waists, and thereby decreases the divergence of the three combined light beams. The decreased divergence allows room for the beam shaping optics 52. The focusing lens 50 is apochromatic to insure that the three larger size (i.e., imaged) circular beam waists are located in a common plane. The three larger size circular beam waists produced by the focusing lens 50 comprise a composite beam waist that constitutes the input to the beam shaping optics 52.

The beam shaping optics 52 includes two cylindrical mirrors 54 and 56. The first cylindrical mirror 54 has power only in the page direction. The second cylindrical mirror 56 has power only in the line direction. In one embodiment, the first cylindrical mirror 54 has concave radius of -119.146 mm in the x-z plane and is tilted in the x-z plane to deviate the single light beam by six degrees. The cylindrical mirror 56 has concave radius of -261.747 millimeters in the y-z plane and is tilted in the y-z plane to restore the light beam direction to the direction that it had prior to impinging on the cylindrical mirror 54. The cylindrical mirror 54 shapes the single light beam 42 so as to form three (essentially coplanar) waists W₁, one for each of the three wavelengths, in the page direction. These three waists are located in the plane 57 at or near the polygon facet 61. (See FIGS. 1a and 6). The cylindrical mirror 56 also shapes the single light beam 42 so as to form three coplanar waists, one for each of the three wavelengths, in the line direction. These three waists W₂ are located in the plane 73 (FIG. 7) approximately one meter away behind the first vertex V₁ of the f-θ lens 70 (see FIG. 4). The sizes and locations of these waists for each of the three wavelengths are provided in the "Beam Shaping and Pyramid Correction" section of the specification. The printer system of the present embodiment is convenient for use with any beam shaping optics producing waists at the locations given in the "Beam Shaping and Pyramid Correction" section of the specification.

As stated above, after being shaped by the shaping optics 52, the single light beam 42 is directed towards the polygon facet 61. This facet 61 is located at or near plane 57. Although a rotating polygon deflector may be used in the invention, other deflectors or scanning means may also be employed, so long as they are capable of deflecting the single light beam by a sufficient amount at the high speed required by the printer system. At the center of a scan line (here defined as 0° polygon rotation), the single light beam's angle of incidence on the polygon facet 61 is 30 degrees. The light beam 42 striking the polygon facet 61 and the single light beam 42 reflected from the polygon facet 61 form a plane which is normal to the direction of the polygon's axis of rotation 63. In other words, the angle of incidence has no component in the page direction.

Upon reflection from the polygon facet 61, the deflected light beam 42 enters the f-θ scan lens 70 as it is being scanned in a plane which is perpendicular to the axis of rotation 63 of the polygon. As described above, this single light beam 42 (also referred as an input beam when discussed in conjunction with the f-θ lens) comprises three coherent coaxial laser light beams having perspective wavelengths of 458 nm, 532 nm, and 685 nm, and has beam characteristics determined by the fiber optic multiplexor 40, focusing lens 50, and the beam shaping mirrors 54 and 56. The f-θ lens 70, illustrated in FIG. 4, includes means for correcting the primary and secondary axial color aberration. The f-θ lens 70 itself is uncorrected for lateral color. Thus red, blue and green spots are separated as shown schematically in FIG. 1b. The overall printer system 10 is corrected for lateral color by modulating the three light beams at three different data rates as later described. The f-θ lens 70 is corrected so that residual lateral color errors (after a linear electronic correction is applied) are insignificant. The detail description as the f-θ lens 70 is provided in the "F-θ Lens" section of this specification.

After passing through the f-θ lens 70, the deflected light beam 42 reflects off a conjugating cylindrical mirror 80 before it impinges on the photosensitive medium 100. (See FIGS. 8a, 8c, 8d). The cylindrical mirror 80 has optical power in X-Z plane (page direction) only (FIG. 8e). The cylindrical mirror 80 corrects for pyramid error of the polygon's facets. This is discussed in more detail in the "Beam Shaping and Pyramid Correction" section of the specification. A piano fold mirror 84 can be placed between the f-θ lens 70 and the cylindrical mirror 80 or between the cylindrical mirror 80 and an image surface 99 in order to place the image surface 99 in a desirable location, where it (at least in line scan direction) coincides with the photosensitive medium 100. Such a fold mirror has no effect on the performance of the printer system. In the preferred embodiment of the present invention, the image surface 99 is a plane.

As stated above, the fiber optic multiplexor 40 produces a beam waist of different size in each of the three colors at the output end of the fiber 40d. Because the f-θ lens 70 is designed to work with the light beam 42 after it has passed through a common apochromatic focusing lens and a common apochromatic beam shaping optics 52, the sizes of the three spots at the image surface 99 will be different for the three wavelengths. The spots at the image surface 99 will maintain the same relative sizes as the three waists located at the output end of the beam combining fiber 40d (of the fiber optic multiplexer 40). This variation in spot size between wavelengths does not significantly impact the perceived image quality.

In the actual embodiment, the radii of the spots produced by the printer system 10 at the image surface 99 at the exp(-2) power level are: 0.035 mm at λ=532 nm, 0.032 mm at λ=457.9 nm, and 0.044 mm at λ=685 nm. As stated above, the image surface 99 of the f-θ lens 70 coincides with the location of the photosensitive medium 100. In this embodiment the photosensitive medium 100 is a conventional photographic paper. The paper rests on a support 100' which moves the paper in a predetermined direction. Writing with spots of this size onto photosensitive medium 100 over a scan line 12 inches long will produce sufficient resolution when the resulting prints are examined at a normal viewing distance. These spots (red, blue, green) refer to the images produced by the light beam on an instantaneous basis. These spots are produced in a series and their location changes with the rotation of the polygon. Each pixel on the page receives up to three spots, one for each color.

Beam Shaping

As discussed in the previous section, the cylindrical mirrors 54 and 56 of the beam shaping optics 52 direct the single light beam 42 containing all three colors toward the polygon facet 61 and cause the single light beam 42 to converge in both the line and page direction (as shown in FIGS. 6 and 7). By "beam shaping optics" we mean beam shaping optics that shape a light beam differentially in the line direction and in the page direction. In this embodiment of the printer system 10, the single light beam 42 converges to a point near the facet 61 in the X-Z or page direction (see FIG. 5), and toward a point approximately one meter behind the front-most vertex V₁ of the f-θ lens 70 in the Y-Z or line direction (see FIG. 7). Thus, the beam shaping optics 52 adjusts the spot size and converges the single light beam 42 by different amounts in the page and line direction. The beam convergence is much faster in the page direction (see FIG. 6) than the line direction (see FIG. 7), this arrangement having been found to allow superior correction of the f-θ lens 70 and other post-polygon optics.

More specifically, in one embodiment, the focusing lens 50 and the beam shaping optics 52 produce a shaped light beam which converges in such a manner as to produce 1.) a green, page direction waist W₁ at a plane located 22.904 mm in front of the first vertex V₁ of the f-θ lens 70 (i.e., this beam waist is located between the polygon facet 61 and the f-θ lens) and 2.) a green, line direction waist W₂ 995.7 mm behind the first vertex V₁ of the f-θ lens 70 (the line direction beam waist is located between the f-θ lens 70 and the image surface 99). The size of the waists may be adjusted by the beam shaping optics depending on the spot size desired at the image surface. For example the exp(-2) power radius of the green waist in the line direction may be 0.114 mm and the exp(-2) power radius of the green waist in the page direction may be 0.0396 mm.

Similarly, the focusing lens 50 and the beam shaping optics 52 produce a shaped light beam 42 which converges in such a manner as to produce 1.) a blue, page direction waist W₁ at a plane located 22.893 mm in front of the first vertex V₁ of the f-θ lens 70 and 2.) a blue, line direction waist W₂ at a plane located 995.8 mm behind the first vertex of the f-θ lens. For example, the exp(-2) power radius of the blue waist in the line direction may be 0.104 mm and the exp(-2) power radius of the blue waist in the page direction may be 0.036 mm.

Similarly, the focusing lens 50 and the beam shaping optics 52 produce a shaped light beam which converges in such a manner as to produce 1.) a red, page direction waist W₁ at a plane located 22.790 mm in front of the first vertex V₁ of the f-θ lens 70 and 2.) a red, line direction waist W₂ at a plane located 995.9 mm behind the first vertex of the f-θ lens. For example, the exp(-2) power radius of the red waist in the line direction may be 0.144 mm and the exp(-2) power radius of the red waist in the page direction may be 0.0495 mm.

Polygon

The f-θ lens 70 of the preferred embodiment is designed to work with a variety of rotating polygons. This makes it particularly suitable for use with a dual format pre-objective scanner of the type described, for example, in the U.S. patent application Ser. No. 08/771,367, filed Dec. 16, 1996 in the name of Michael Harrigan, Badhri Narayan and Drew Summers which is incorporated by reference herein. It is particularly suitable for use with 10 facet polygons having an inscribed radius between 32.85 mm and 40.709 mm. These polygons are rotated by +/-13.5 degrees to produce a scan line 12 inches long at the image surface 99.

The f-θ lens 70 also works well with 24 facet polygons having an inscribed radius between 38.66 mm and 44 mm. These polygons are rotated by +/-5.625 degrees to produce scan lines 5 inches long at the image surface 99.

F-θ Lens

The lens 70 is arranged in the optical path of the printer system 10 as shown in FIGS. 8a-8d.

As shown in FIG. 4, the optical axis O. A. of the f-θ lens 70 extends in a direction referred to herein as the Z direction. When the polygon rotates (for line scanning) the light beam 42 is scanned through the Y-Z meridian. (See FIGS. 9a-9c). The cross-scan (also referred to as the page direction) is in the X direction.

The f-θ lens 70 described herein is particularly suitable for use in the laser printer system 10. Due to the lateral color present in the f-θ lens 70, the printer system 10 simultaneously produces three spatially separated scanning spots at the image surface 99. Each of the three spots contains energy in one of the three laser wavelengths. This separation is compensated for in a manner described in the "Lateral Color Correction" section of this specification. To summarize, the spots are properly superimposed on a photosensitive medium when the data rates at which the three light beams are modulated are linearly adjusted to compensate for the lateral color of the f-θ lens 70.

Ideally, the lateral color should be completely corrected with no residual errors by using three different data rates to move data between the digital image store and the laser modulator control circuitry. The spots should ideally travel in a straight line, at uniform velocities (as the polygon is rotated with uniform angular velocity), and should not significantly change their size and shape as they travel down the line. If necessary, the variation in the spot velocities can be compensated for by adjusting the data rate as the spots move across the scan line. The spots should have approximately circular shapes, with energy distributions which are approximately gaussian. The spot diameter at the exp(-2) level should be about 60-105 μm (in green light) in order to achieve sufficient resolution at the photosensitive medium, the smaller size being necessary to achieve overprinting of fine text on a picture. It is preferred that this spot diameter be 64-88 μm.

A further requirement of an f-θ scan lens 70 of the preferred embodiment is that it be readily manufacturable at a reasonable cost. This requires that the lens have spherical surfaces on relatively low cost glass.

The f-θ lens 70 satisfies all of the above requirements. In FIGS. 4 and 8a there is shown the f-θ lens 70 which is constructed in accordance with the present invention. In the present embodiment of the present invention, the f-θ lens includes four lens components arranged along an optical axis. They are: a first lens component 72 of negative optical power, a second lens component 74 of positive optical power, a third lens component 76 of negative optical power, and a fourth lens component 78 of positive optical power.

The lens components satisfy the following relationships:

-1.6<f₁ /f<-0.9;

0.38<f₂ /f<0.5;

-0.65<f₃ /f<-0.50;

0.73<f₄ /f<0.9,

where f₁ is the focal length of the first lens component, f₂ is the focal length of the second lens component, f₃ is the focal length of the third lens component, f₄ is the focal length of the fourth lens component, and f is the focal length of the f-θ lens 70. The lens component 72 is a meniscus negative element, concave toward the polygon side. Lens component 74 is a meniscus positive lens element, also concave toward the polygon. Lens component 76 is a meniscus negative lens element, concave toward the image surface 99. Lens component 78 is a meniscus positive lens element, also concave toward the image surface 99. In the exemplary f-θ lens 70, the lens elements are formed of Schott glass with the lens element 72 being an PK-51A type, the lens element 74 being LAK-21 glass, the lens element 76 being an SFL-56 glass, and the lens element 78 being an F-2 type glass. The f-θ lens 70 is apochromatic, that is, it is corrected for both the primary and the secondary axial color at a wavelength of 458 nm, 532 nm and 685 nm. In this embodiment, the first lens component 72 is a single lens element satisfying the following equations:

    Vd.sub.1 >65; and

    P.sub.g,F;1 >0.53,

where Vd₁ is the V-number of the first lens component material and P_(g),F;1 is its relative partial dispersion.

The details of the elements in lens 70 are shown in TABLE 1A.

In this table, the radii of curvature (r1-r8) and thicknesses of the elements are in millimeters.

                  TABLE 1A                                                         ______________________________________                                                           THICK-                                                       SURF     RADIUS   NESS      INDEX    V NUMBER                                  ______________________________________                                         Entrance Pupil    24.00     Polygon facet                                      1        -33.0678 10.634    1.529    77.0                                      2        -44.642  0.925     AIR                                                3        -341.050 7.654     1.641    60.1                                      4        -85.6131 0.836     AIR                                                5        423.736  12.550    1.785    26.1                                      6        129.480  6.034     AIR                                                7        139.081  19.689    1.620    36.4                                      8        403.727                                                               ______________________________________                                    

The following tables 1B-1D show the f-θ compliance and the relative spot velocity achieved in the green, red and blue light for the f-θ lens when used with a 10 facet polygon having an inscribed radius of 32.85 mm.

                                      TABLE 1B                                     __________________________________________________________________________     F-Theta compliance and instantaneous spot velocity data:                        = 532                                                                         CFG                                                                               ROT IDEAL                                                                               ACTUAL                                                                              DELTA                                                                               PERCENT                                                                              REL                                                LOG10                                                                          NBR                                                                               ANGLE                                                                              RAYHT                                                                               RAYHT                                                                               RAYHT                                                                               ERROR VEL REL VEL                                        __________________________________________________________________________     1  0.000                                                                              0.000                                                                               0.000                                                                               0.000                                                                               0.000 1.0000                                                                             0.000                                          2  4.500                                                                              -51.265                                                                             -50.089                                                                             1.175                                                                               -2.293                                                                               1.0104                                                                             -0.0045                                        3  9.000                                                                              -102.530                                                                            -101.282                                                                            1.248                                                                               -1.217                                                                               1.0440                                                                             -0.0187                                        4  13.500                                                                             -153.794                                                                            -154.644                                                                            -0.850                                                                              0.553 1.0948                                                                             -0.0393                                        5  -4.500                                                                             51.265                                                                              50.149                                                                              -1.116                                                                              -2.176                                                                               1.0129                                                                             -0.0056                                        6  -9.000                                                                             102.530                                                                             101.526                                                                             -1.004                                                                              -0.979                                                                               1.0492                                                                             -0.0208                                        7  -13.500                                                                            153.794                                                                             155.209                                                                             1.415                                                                               0.920 1.1023                                                                             -0.0423                                        __________________________________________________________________________

                                      TABLE 1C                                     __________________________________________________________________________      = 457.9                                                                       CFG                                                                               ROT IDEAL                                                                               ACTUAL                                                                              DELTA                                                                               PERCENT                                                                              REL                                                LOG10                                                                          NBR                                                                               ANGLE                                                                              RAYHT                                                                               RAYHT                                                                               RAYHT                                                                               ERROR VEL REL VEL                                        __________________________________________________________________________     1  0.000                                                                              0.000                                                                               0.000                                                                               0.000                                                                               0.000 1.0000                                                                             0.0000                                         2  4.500                                                                              -51.237                                                                             -50.059                                                                             1.179                                                                               -2.300                                                                               1.0105                                                                             -0.0045                                        3  9.000                                                                              -102.474                                                                            -101.224                                                                            1.251                                                                               -1.221                                                                               1.0441                                                                             -0.0188                                        4  13.500                                                                             -153.712                                                                            -154.561                                                                            -0.849                                                                              0.552 1.0949                                                                             -0.0394                                        5  -4.500                                                                             51.237                                                                              50.119                                                                              -1.118                                                                              -2.183                                                                               1.0130                                                                             -0.0056                                        6  -9.000                                                                             102.474                                                                             101.470                                                                             -1.005                                                                              -0.981                                                                               1.0494                                                                             -0.0209                                        7  -13.500                                                                            153.712                                                                             155.132                                                                             1.420                                                                               0.924 1.1025                                                                             -0.0424                                        __________________________________________________________________________

                                      TABLE 1D                                     __________________________________________________________________________      = 685                                                                         CFG                                                                               ROT IDEAL                                                                               ACTUAL                                                                              DELTA                                                                               PERCENT                                                  LOG10                                                                          NBR                                                                               ANGLE                                                                              RAYHT                                                                               RAYHT                                                                               RAYHT                                                                               ERROR VEL REL VEL                                        __________________________________________________________________________     1  0.000                                                                              0.000                                                                               0.000                                                                               0.000                                                                               0.000 1.0000                                                                             0.0000                                         2  4.500                                                                              -51.321                                                                             -50.145                                                                             1.177                                                                               -2.293                                                                               1.0104                                                                             -0.0394                                        3  9.000                                                                              -102.643                                                                            -101.393                                                                            1.250                                                                               -1.218                                                                               1.0440                                                                             -0.0187                                        4  13.500                                                                             -153.964                                                                            -154.816                                                                            -0.851                                                                              0.553 1.0950                                                                             -0.0045                                        5  -4.500                                                                             51.321                                                                              50.205                                                                              -1.117                                                                              -2.176                                                                               1.0129                                                                             -0.0056                                        6  -9.000                                                                             102.643                                                                             101.637                                                                             -1.005                                                                              -0.980                                                                               1.0491                                                                             -0.0208                                        7  -13.500                                                                            153.964                                                                             155.381                                                                             1.417                                                                               0.920 1.1025                                                                             -0.0424                                        __________________________________________________________________________

If necessary, the variation in the spot velocities can be compensated for by adjusting the rate at which data in the digital image store (described in the "Lateral Color Correction" section) is moved to the circuitry controlling the laser modulators. The adjustment amount is the same for each of the modulators.

The following Table 2 shows how the spots grow as the polygon is rotated and the spot moves across the scan line. This data is for a 10 facet polygon having an inscribed radius of 32.85 mm. A polygon rotation of ±13.5 degrees corresponds to a scan position of approximately ±6 inches at the image surface 99.

                                      TABLE 2                                      __________________________________________________________________________      ##STR1##                                                                        spot sizes (mm) at image surface computed from astigmatic Gaussian             beam                                                                         trace using the following                                                       ##STR2##  waist radii at an object plane:                                      = 532, ω = .00189;  = 457.9, ω = .00172;  = 685, ω =        .00237                                                                         Effects of beam truncation are not included in this computation                POLYGON                                                                        ROTATION                                                                              13.500°                                                                     9.000°                                                                       4.500°                                                                      0.000°                                                                       -4.500°                                                                     -9.000°                                                                      -13.500°                              __________________________________________________________________________      = 532 ωy                                                                       0.0390                                                                             0.0371                                                                              0.0359                                                                             0.0355                                                                              0.0359                                                                             0.0371                                                                              0.0390                                       ωx                                                                              0.0359                                                                             0.0355                                                                              0.0353                                                                             0.0352                                                                              0.0353                                                                             0.0356                                                                              0.0358                                        = 457 ωy                                                                       0.0360                                                                             0.0340                                                                              0.0328                                                                             0.0325                                                                              0.0328                                                                             0.0340                                                                              0.0357                                       ωx                                                                              0.0329                                                                             0.0324                                                                              0.0322                                                                             0.0322                                                                              0.0323                                                                             0.0325                                                                              0.0328                                        = 685 ωy                                                                       0.0490                                                                             0.0467                                                                              0.0452                                                                             0.0450                                                                              0.0452                                                                             0.0467                                                                              0.0489                                       ωx                                                                              0.0447                                                                             0.0443                                                                              0.0441                                                                             0.0441                                                                              0.0442                                                                             0.0444                                                                              0.0446                                       __________________________________________________________________________      where ωy = line direction                                                ##STR3##                                                                      spot radius and ωx = page direction spot radius   Pyramid Error           Correction

Scanning systems using a rotating polygon deflectors are subject to an image defect known as banding, which is most easily seen in areas of the image where it is free of subject detail, i.e., a blank wall or a cloud free sky scene. Light and dark bands, which are not part of the desired image, will appear in these areas. These bands are caused by repetitive non-uniform spacing of the scan lines. The banding is caused by a facet, or facets, on the polygon which are tilted slightly out of position. Thus, every time the facet which is out of position comes around, it will cause the light beam to move ever so slightly out of the nominal light beam plane--i.e., the light beam plane formed by a rotating light beam in the absence of any pyramid error. After going through the f-θ lens, this misplaced light beam will land in a slightly different position on the image surface, generating what is known as "cross-scan" error, since the position error is in a direction which is perpendicular to the scan line. An f-θ lens must function with the other optical elements in the printer system to produce an image which is free from banding when a "good" polygon is used, that is, a polygon in which pyramidal angle errors on the polygon facets do not exceed +/-10 arc seconds, as measured with respect to the axis of rotation of the polygon.

In an embodiment of the present invention, the pyramid error is corrected by keeping the polygon facet 61 conjugate with the image surface 99 in the page meridional (X-Z plane). (Conjugate points are defined herein as any pair of points such that all rays from one are imaged on the other within the limits of validity of gaussian optics). This conjugation is achieved by the conjugating cylindrical mirror 80 working in conjunction with f-θ lens 70. Thus, there is a focal point (beam waist) at both the polygon facet 61 and at the photosensitive medium 100, and the polygon facet is thereby conjugated to the photosensitive medium 100. As a result, if the polygon facet 61 is tilted slightly in the X-Z plane, that is, around the "object" point, the path of the rays through the printer system 10 is slightly different from that shown in the figure, but the rays all go to the same "image" point, and the cross-scan error is zero.

The conjugation condition described above imposes requirements on the beam shaping optics. Conjugation of the polygon facet 61 and the image surface 99 in the page direction implies that in the page direction, a beam waist (for each wavelength) is located at (or adjacent to) both locations (i.e., at or near the polygon facet 61, and at or near the image surface 99). Hence, the beam shaping optics 52 must produce a beam waist W₁ in the page direction at or near the polygon facet 61. This is achieved in the current design as is discussed in the "Beam Shaping" section and is shown in FIG. 6. It is preferred that the beam waist in the page direction be located less than ##EQU1## from the polygon facet 61 (where f is the focal length of the f-θ lens).

The degree of convergence (of the single light beam 42) in the line direction is not similarly constrained. In the present embodiment, the beam shaping optics 52 converge the single light beam 42 in the line direction to form a beam waist behind the rear focal point of the f-θ lens 70. It is preferred that the beam waist W₂ in the line direction at a distance be at least 1/3 f behind the first vertex V₁ of the θ lens 70 (see FIG. 7). In the printer system 10 the distance between the rear focal point of the f-θ lens and the waist location is approximately equal to the focal length of the f-θ lens 70. More specifically, the f-θ lens 70 has a focal length of 426.4 mm and the line direction waist formed by the beam shaping optics 52 is located 488.9 mm behind the rear focal point. This arrangement has optics, as well as providing a compact system.

The conjugating cylindrical mirror 80 (see FIG. 8e) is located between the f-θ lens 70 and the photosensitive medium 100. As stated above, it corrects for the pyramid error of the polygon facets by conjugating, in the X-Z plane, the polygon facet 61 with the image surface 99. This cylindrical mirror 80 has a concave radius (in the page direction) of 190.500 mm and is located 153.053 mm behind the last vertex of the f-θ lens. The cylindrical mirror 80 is tilted by 7 degrees and deviates the single light beam 42 by 14 degrees. The image surface 99 is located 162.96 mm behind the cylindrical mirror 80, the distance being measured along the deviated light beam. As mentioned above, various plano fold mirrors 84 may be placed in the scanning system without affecting its performance.

FIGS. 9a, 9b, 9c shows the position of the light beam 42 on the photosensitive medium 100 (located at the image surface 99) for polygon rotations of +13.5, 0, and -13.5 degrees respectively. This represents scan angles of +27, 0, and -27 degrees, respectively.

More specifically, in Table 3, the computed cross-scan image displacements for the chief (central) rays of the light beam (at wavelengths of 532 nm, 457 nm and 685 nm) are tabulated. It will be seen that the cross-scan displacements are certainly well within acceptable limits.

Table 3 shows the cross-scan displacement due to 10 arc seconds of pyramid error on polygon facet. The displacement units are micrometers.

                  TABLE 3                                                          ______________________________________                                         CROSS-SCAN DISPLACEMENT                                                        POLYGON FIELD                                                                  ROTATION                                                                               ANGLE     = 532 nm   = 457 nm                                                                               = 685 nm                                  ______________________________________                                         4.5°                                                                            9.0°                                                                             -0.0204568 -0.0103607                                                                             -0.0299763                                 9.0°                                                                            18.0°                                                                            -0.0210595 -0.0113009                                                                             -0.0301466                                 13.5°                                                                           27.0°                                                                            -0.0327880 -0.0235740                                                                             -0.0411589                                 -4.5°                                                                           -9.0°                                                                            -0.0189723 -0.0079102                                                                             -0.0294039                                 -9.0°                                                                           -18.0°                                                                           -0.0209200 -0.0091726                                                                             -0.0318579                                 -13.5°                                                                          -27.0°                                                                           -0.0465809 -0.0344084                                                                             -0.0576246                                 none    0.0°                                                                             -0.0202603 -0.0097542                                                                             -0.0302057                                 ______________________________________                                    

Axial Color Aberration

There are two kinds of color aberrations in any lens system: axial color and lateral color. Axial color causes light of different wavelengths to come to a focus at different distances from the rear surface of the lens system. Since axial color is a focus-related phenomenon, it is caused not only by aberrations in a lens system itself but also by the vergence of the input light beam to the lens system.

A fiber optic multiplexor combining the three beams in a three-color laser printer is described in U.S. Pat. No. 5,666,447, issued Sep. 9, 1997, entitled USING OPTICAL FIBER MULTIPLEXER TO COMBINE LIGHT BEAMS IN A LASER PRINTER, by Chih-Li Chuang & Sanwal Sarraf. After the combination by the fiber optic multiplexor, the single light beam 42 passes through a common optical system comprising focusing optics, beam shaping optics, polygon, f-θ lens, cylindrical conjugating mirror, and optional piano fold mirrors until it reaches the image surface.

In the printer system 10, the line direction vergence of the green, blue, and red light beams cannot be adjusted independently because the beam shaping optics 52 is common to the three (combined) light beams. This makes the correction of the axial color in the optical system more difficult. For the printer system 10, the axial color must be corrected when the three light beams have essentially the same vergence. This is what has been done in the f-θ lens 70, as is shown in the OPD plots in FIG. 10, which correspond to f-θ lens performance at the center of the line scan. The construction of the f-θ lens 70 is disclosed in the "F-θ lens" section of the application.

The axial color of the optical system in the page direction must be corrected between the polygon facet and the image surface, in order to prevent color banding due to pyramid errors. Otherwise, the pyramid error will only be corrected in a single color. In the printer system 10, the axial color is corrected in both meridians, all the elements are spherical, a costly cemented cylindrical doublet is unnecessary, and the pyramid error is corrected with the conjugating cylindrical mirror 80.

Lateral Color Correction

As stated previously, the lateral color aberration of the F-θ lens 70 is uncorrected. Lateral color is the variation in image height of focused spots having different wavelengths, or colors, taken in a specified image surface (see FIG. 1b). For example, in normal photographic objectives for use in color photography, lateral color is typically measured by Y'(at λ₁ =486.1 nm)-Y'(at λ₂ =656.3 nm); this is the difference in image height, taken in the gaussian focal plane for λ=546.1 nm, between the blue point image and the red point image. Lateral color, as opposed to axial color, only occurs away from the optical axis, out in the field of the lens. Usually, the farther away from the axial image point, the greater the amount of lateral color. Thus, the largest amount of lateral color often occurs near the edge of the field of view of the lens. In the printer system 10, the lateral color is exhibited as a separation of red, blue and green spots along the scan line at the photosensitive medium (FIG. 1b).

The lateral color in the printer system 10 is corrected by modulating the three light beams at three different data rates. To understand this, consider the following hypothetical example: Suppose that the lateral color in an f-θ lens is such that for a given amount of polygon rotation the green light beam intercepts the image surface at a location 100 pixels high whereas the red light beam intercepts the image surface at a location 101 pixels high and the blue light beam intercepts the image surface at a location 99 pixels high (see FIG. 11). For example, if the printer worked at 512 dots per inch, the blue and green spots would be separated by a distance d₁ =1/512 inch and the red and green spots would be separated by a distance d₂ =1/512 inch. According to one embodiment of the invention, the rate at which data is moved from a digital image store to the circuitry controlling the laser modulators is determined by three data clocks C₁ -C₃ shown in FIG. 1a. One clock controls the data rate for the green channel, a second clock controls the data rate for the blue channel, an a third clock controls the data rate for the red channel. If these three clocks are run at the same rate, then, at any instant in time, the three laser intensities correspond to the required green, blue and red intensity values for the same pixel. Due to the spot separation (d₁, d₂) produced at the image surface 99 by the lateral color in the f-θ lens, the image recorded on the photosensitive medium will show color fringing at an image location of 100 pixels high. More specifically, there will be color fringing of two pixels between red and blue, one pixel between green and red and one pixel between green and blue.

Now suppose that the blue data clock is run at a frequency (i.e., data rate) f_(B) which is 99% of the green clock frequency f_(G) and that the red clock is run at a frequency f_(R) which is 101% of the green clock frequency. At the given amount of polygon rotation, the green light beam will intercept the image surface at a location 100 pixels high and the modulation of the light beam is appropriate to produce the exposure of the 100th pixel. Likewise, at this same polygon rotation, the red light beam still intercepts the image surface at a location 101 pixels high. However, since the red clock is being run at 101% of the frequency of the green clock, the red light beam is now correctly data modulated to give the proper exposure for the 101st pixel. Similarly the blue light beam remains 99 pixels high, but the blue laser light is data modulated to give the proper exposure for the 99th pixel. That is, at any given time (or at any given polygon rotation position) the laser printer 5 may produce three color spots, but the image information contained in each one of the three color beams is different--i.e., it corresponds to different pixels. So at same time T₁, pixel 98 will receive the red beam R, at time T₁ +Δ the pixel 98 will receive the green beam G, and in time T₁ +2Δ it will receive the blue beam B (FIG. 12). This way, when the printer is operating in locations other than the center of the line scan, each pixel can receive red, green and blue image modulated light, albeit at a different time. Therefore, there will be no color fringing at the 100th pixel. Thus, in the printer system 10, the data rates f_(B), f_(G) and f_(R) are not the same. More specifically, the data rates are f_(B) =k₁ ×f_(G), f_(R) =k₂ ×f_(G), where k₁ and k₂ are constants chosen to compensate for spot separation during the line scan.

In any laser printer system, there is a detection procedure to determine a specific starting location for each line on the photosensitive medium. In a printer system 10, this is done by utilizing a "split" (dual) detector and the (unmodulated) red light beam to generate the initial start up pulse. More specifically, the split detector detects the presence of the light beam and from its location (with respect to the beginning of the line), determines the time delays needed for starting of the modulation of each of the three light beams, so that the appropriate pixel at the beginning of the line scan is exposed with the light beam carrying the proper data information.

A potential problem remains that the same clock rates which produced good results for an image height of 100 pixels might still produce color fringing at other image heights. However, in the printer system 10, these residual lateral color errors have been corrected in the f-θ lens 70 so that the worst residual error (due to the lateral color aberration) over the entire scan line is less than 20% of the size of a green pixel. This is shown in tables 2 and 4. Table 2 shows the spot size across the scan line. Table 4 shows the residual lateral color when the light beams are modulated at the rates shown at the bottom of the table. Both of these tables are for a 10 facet polygon with an inscribed radius of 32.85 mm. Similar results hold for the other 10 facet polygon sizes. The results for the 24 facet polygons are much better.

                  TABLE 4                                                          ______________________________________                                         Difference in line direction image position (in millimeters) for red,          green and blue colors with red, green and blue pixel clocks in drive           electronics adjusted in the ratio of 1.0011:1.0000:0.99946                               ( = 457) - ( = 532)                                                                          ( = 685) - ( = 532)                                    ROT       Residual Error                                                                               Residual Error                                         ANGLE     (Blue-Green)  (Red-Green)                                            ______________________________________                                         4.500     0.003         0.001                                                  9.000     0.003         0.003                                                  13.500    0.001         -0.002                                                 -4.500    -0.003        -0.001                                                 -9.000    -0.001        -0.002                                                 -13.500   0.006         0.002                                                  ______________________________________                                          Green  = 532 nm; Blue  = 457.9 nm; Red  = 685 nm                         

In a laser printer of a type which can incorporate the f-θ lens of the present invention, the system parameters can be as follows:

Wavelengths: 532, 457.9, and 685 nm

Scan length: 12 inches

Polygon Duty Cycle: 0.75

Polygon inscribed radius: 32.85 through 40.709

Number of polygon facets: 10

Total Scan angle: 54 degrees. (+/-27 degrees with respect to the optical axis;

+/-13.5 degrees of polygon rotation)

Light beam input angle onto polygon facet: 60 degrees from optical axis of

f-θ lens (30 degree angle of incidence on polygon facet)

Desired gaussian beam radius at the exp(-2) power point: 0.035 mm at λ=532 nm.

In a laser printer of a type which incorporates the f-θ lens 70 of the present invention, the system parameters can also be as follows:

Wavelengths: 532, 457.9, and 685 nm

Scan length: 5 inches

Polygon Duty Cycle: 0.75

Polygon inscribed radius: 38.66 through 44.00

Number of polygon facets: 24

Total Scan angle: 22.5 degrees. (+/-11.25 degrees with respect to the optical axis; +/-5.625 degrees of polygon rotation)

Light beam input angle onto polygon facet: 60 degrees from optical axis of f-θ lens (30 degree angle of incidence on polygon facet)

Desired gaussian beam radius at the exp(-2) power point: 0.051 mm at 532 nm.

As stated above, the f-θ lens 70 itself is not corrected for lateral color. Correction of the lateral color in the scanner requires running the green, blue, and red clocks modulating the lasers in the ratio 1:000: 0.99946: 1.0011.

As disclosed in the "Axial Color Aberration" section of this specification, the f-θ scan lens 70 by itself is corrected for primary and secondary axial color. This is a requirement for this type of scanner because the beam shaping optics 52 is common to the three light beams. In the X-Z direction, the f-θ scan lens conjugates the polygon facet to the image surface (in all three wavelengths), this requires the use of an auxiliary cylindrical mirror having power in only the X-Z direction. Assuming the "object" is at the polygon facet, the axial color in the X-Z direction for the f-H lens 70 is zero; it is also zero for the cylindrical mirror and, hence, the conjugation holds at all three wavelengths.

The invention has been described in detail with particular reference to the embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. For example, other laser sources producing red, green and blue light beams in wavelengths other than 458 nm, 532 nm or 685 nm may be also utilized. Changing the wavelengths will change the ratios between the corresponding data rates.

    ______________________________________                                         PARTS LIST                                                                     ______________________________________                                         Printer system        10                                                       Light beam            12, 14, 16                                               3 laser sources       22, 24, 26                                               3 modulators          21, 34, 36                                               Beam combiner         40                                                       Single light beam     42                                                       Focusing lens         50                                                       Beam shaping optics   52                                                       Light deflector (polygon)                                                                            60                                                       Polygon Facet         61                                                       f-θ lens        70                                                       four lens components  72, 74, 76, 78                                           Cylindrical mirror    80                                                       Flat mirror           84                                                       Processor unit        90                                                       means for reading     92                                                       means for controlling 94                                                       Image surface         99                                                       Photosensitive medium 100                                                      Support               100'                                                     ______________________________________                                     

What is claimed is:
 1. A printer system, said printer system comprising:(a) three light sources, generating respective light beams of different wavelengths; (b) three modulators, each of said modulators modulating an intensity of each of said light beams by image data, thereby providing three modulated light beams; (c) a beam combiner combining said three modulated light beams into a single light beam; (d) a single set of beam shaping optics, shaping said single light beam into a shaped light beam, said shaped light beam having different vergences and waist locations in a page direction and a line direction, said vergences and said waist locations being substantially the same in each of said three wavelengths; (e) a light deflector deflecting the shaped light beam and performing a scanning function, thereby providing a deflected light beam; and (f) a single f-θ lens focusing said deflected light beam onto an associated image surface.
 2. A printer system, as defined in claim 1, wherein said single light beam is a combination of (a) a first light beam having a wavelength of about 532 nm, (b) a second light beam having a wavelength of about 457.9 nm, and (c) a third light beam having a wavelength of about 685 nm.
 3. A printer system according to claim 1 wherein:said light deflector is a polygon having a plurality of reflective facets, at least one of said facets facing a front-most surface of said f-θ lens; and said single set of beam shaping optics producing (i) a beam waist in the page direction adjacent to an associated one of said plurality of reflective facets, and (ii) a beam waist in the line direction at a distance of at least 1/3 f behind a first vertex of said f-θ lens, where f is a focal length of said f-θ lens.
 4. A printer system according to claim 3, wherein said beam waist in the page direction is located at a distance that is less than ##EQU2## from the associated reflective facet, where f is the focal length of said f-θ lens.
 5. A printer system according to claim 1 wherein(a) said f-θ lens introduces lateral color aberration, and (b) said modulators are modulating said light beams at three data rates, different from one another, and data rate ratio is being used to compensate for the lateral color aberration produced by the f-θ lens.
 6. A printer system for printing on an associated photosensitive medium, said printer comprising:(a) an image information store, said store containing three values for each pixel in an image to be created, each of said three values representing a required level of intensity of light in one of three required wavelengths, said wavelengths matching a spectral sensitivity of an associated photosensitive medium so as to produce full color images when impinging on said associated photosensitive medium; (b) a processor processing said values; (c) three laser light sources each generating a light beam, each of said light beams being a respective one of said required wavelengths; (d) three modulators driven by said processor, each of said modulators modulating intensity of one of said light beams by image data and together providing three modulated light beams; (e) a beam combiner combining said three modulated light beams into a single light beam; (f) a single set of beam shaping optics shaping said single light beam into a shaped light beam, said shaped light beam having different vergences and waist locations in a page direction and a line direction, said vergences and said waist locations being substantially the same in each of the three wavelengths; (g) a light deflector deflecting said shaped light beam and performing a scanning function, thereby providing a deflected light beam; and (h)) a single, axial color achromatic f-θ lens (i) intercepting the deflected light beam, and (ii) focusing the deflected light beam into three spots of different wavelengths on the associated photosensitive medium.
 7. A printer system according to claim 6, wherein said processor includes means for (a) determining for each position of the deflector where said spots will appear on the associated photosensitive medium; (b) selecting pixels from the image information store that correspond to these spots; and (c) in combination with said three modulators modulates said light beams.
 8. A printer system according to claim 6 further comprising:a support means supporting said associated photosensitive medium in a flat plane and a conjugating cylindrical mirror (a) having a concave radius in page direction, (b) being located between said f-θ lens and said associated photosensitive medium, (c) receiving said single light beam from said f-θ lens, and (d) directing said single light beam onto the associated photosensitive medium.
 9. A printer system comprising:(i) at least two light sources, each of said light sources producing a light beam of a different wavelength; (ii) at least two modulators, one for each light beam, said modulators modulating said light beams with image data and together providing at least two modulated light beams; (iii) a combiner combining said modulated light beams into a single light beam having at least two coaxial components of different wavelengths; (iv) a single set of beam shaping optics shaping said single light beam into a shaped light beam; (v) a light deflector deflecting said shaped light beam, thus providing a deflected light beam; (vi)) a single f-θ lens (a) intercepting said deflected light beam and(b) focusing the deflected light beam on a photosensitive medium.
 10. A printer system according to claim 9 having three light sources producing three light beams of different wavelengths and having three modulators for modulating said three light beams.
 11. A printer system according to claim 10, wherein said single light beam is a combination of (a) a first light beam having a wavelength of about 532 nm, (b) a second light beam having a wavelength of about 457.9 nm, and (c) a third light beam having a wavelength of about 685 nm.
 12. A printer system according to claim 10 wherein said combiner is a fiber optic multiplexor.
 13. A printer system according to claim 10 wherein said beam shaping optics in combination with said combiner is producing a single light beam that has three collinear components.
 14. A printer system according to claim 10 wherein vergence of the single light beam in page direction is stronger than in the line direction.
 15. A printer system according to claim 10 wherein said light deflector is a polygon having a plurality of reflective facets, at least one of said facets facing a front-most surface of said f-θ lens; and said beam shaping optics produces a page direction beam waist in the vicinity of said one polygon facet and a line direction beam waist located between the f-θ lens and the image surface of the f-θ lens. 